In general, liquid crystal display devices are mounted in personal computers, wordprocessors, pachinko machines, vehicle navigation systems, small-size TV sets, and the like, and have recently been in increasing demand. However, liquid crystal display devices are expensive, and hence demand for cost reduction has increased year by year. Of the components of a liquid crystal display device, a color filter exhibits a high cost ratio, and demand for a reduction in the cost of the color filter has increased.
A color filter used in a liquid crystal display device is formed by arraying filter elements colored in, for example, red (R), green (G), and blue (B) on a transparent substrate. A black matrix (BM) for blocking light is provided around each filter element to improve the display contrast of the liquid crystal display device. BMs range from a BM using a Cr metal thin film to a recent resin BM using a black resin.
An overcoat layer (protective layer) made of an acrylic-based resin or epoxy-based resin and having a thickness of 0.5 to 2 μm is formed on a colored layer including a filter element to, for example, improve smoothness. A transparent electrode (ITO) film is further formed on this overcoat layer.
Various conventional methods of coloring the filter elements of a color filter are known, including, for example, a dyeing method, pigment dispersion method, electrodeposition method, and printing method.
In the dyeing method, a water-soluble polymer material as a dyeing material is formed on a glass substrate and patterned into a predetermined shape by photolithography. The obtained pattern is dipped in a dyeing solution. This process is repeated for R, G, and B to obtain color filters.
In the pigment dispersion method, a pigment-dispersed photosensitive resin layer is formed on a transparent substrate by a spin coater or the like. The resultant layer is then patterned. This process is performed once for each of R, G, and B, i.e., repeated a total of three times for R, G, and B, thereby obtaining R, G, and B color filters.
In the electrodeposition method, a transparent electrode is patterned on a substrate, and the resultant structure is dipped in an electrodeposition coating fluid containing a pigment, resin, electrolyte, and the like to be colored. This process is repeated for R, G, and B to form color filters.
In the printing method, a thermosetting resin in which a pigment-based coloring material is dispersed is colored by offset printing. This process is repeated for R, G, and B to form color filters.
The above color filter manufacturing methods have a common feature that the same process must be repeated three times to color layers in three colors, i.e., R, G, and B, and hence the cost is high. In addition, since a large number of processes are required, the yield decreases.
In order to eliminate these drawbacks, color filter manufacturing methods using an ink-jet system are disclosed in Japanese Patent Laid-Open Nos. 59-75205, 63-235901, and 1-217320. The ink-jet system is a method of forming filter elements by injecting coloring materials containing R, G, B color materials onto a transparent substrate using an ink-jet head and drying/fixing the coloring materials. In this method, since R, G, and B portions can be formed at once, simplification of the manufacturing process and a reduction in cost can be achieved. In addition, since the number of steps is smaller than those in the dyeing method, pigment dispersion method, electrodeposition method, printing method, and the like, an increase in yield can be achieved.
In a color filter used in a general liquid crystal display device, black matrix opening portions (i.e., pixels) for partitioning the respective pixels are rectangular, whereas ink droplets discharged from an ink-jet head are almost circular. It is therefore difficult to discharge ink in an amount required for one pixel at once and uniformly spread the ink in the entire opening portion of the black matrix. For this reason, a plurality of ink droplets are discharged to one pixel on a substrate to color it while the ink-jet head is scanned relative to the substrate.
As variations in the amounts of ink filled in the respective pixels are small, a high-quality color filter with reduced unevenness can be manufactured.
The amount of ink discharged from an ink-jet head may vary among nozzles even in discharge driving operation under the same discharge driving condition owing to variations in the structures of nozzles constituting the head or structures associated with discharging operation, driving mechanisms, and driving characteristics. In this case, even if the same numbers of ink droplets are discharged to the respective pixels, the amounts of ink filled in the respective pixels vary because of the use of different nozzles. The variations in the amounts of ink filled lead to unevenness among the pixels, resulting in reductions in the quality and yield of color filters.
In order to solve this problem of density unevenness, the following two methods (bit correction and shading correction) have been adopted. Consider here an ink-jet head for discharging ink using heat energy.
A method (to be referred to as bit correction hereinafter) of correcting the differences in ink discharge amount between the respective nozzles of an ink-jet head IJH, which has a plurality of ink discharge nozzles shown in FIGS. 16 to 18 as disclosed in Japanese Patent Laid-Open No. 9-281324, will be described first.
First of all, as shown in FIG. 16, ink is discharged from, for example, three nozzles, i.e., nozzle 1, nozzle 2, and nozzle 3, of the ink-jet head IJH onto a predetermined substrate P, and the sizes of ink dots formed on the substrate P by the ink discharged from the respective nozzles are detected, thereby measuring the amounts of ink discharged from the respective nozzles. In this case, the width of a heat pulse applied to the heater of each nozzle is kept constant, and the width of a pre-heat pulse is changed. With this operation, a curve like the one shown in FIG. 17 can be obtained, which represents the relationship between the pre-heat pulse width and the ink discharge amount. Assume that all the amounts of ink discharged from the respective nozzles are to be unified to 20 ng. In this case, it is obvious from the curve shown in FIG. 17 that the width of a pre-heat pulse applied to nozzle 1 is 1.0 μs; to nozzle 2, 0.5 μs; and to nozzle 3, 0.75 μs. By applying pre-heat pulses with these widths to the heaters of the respective nozzles, all the amounts of ink discharged from the respective nozzles can be unified to 20 ng, as shown in FIG. 18. Correcting the amounts of ink discharged from the respective nozzles in this manner will be referred to as bit correction.
FIGS. 19 and 20 are views showing a method (to be referred to as shading correction hereinafter) of correcting density unevenness in the scanning direction of the ink-jet head by adjusting the ink discharge density from each ink discharge nozzle. Assume that as shown in FIG. 19, when the amount of ink discharged from nozzle 3 of the ink-jet head is set as a reference, the amount of ink discharged from nozzle 1 is −10%, and that from nozzle 2 is +20%. In this case, while the ink-jet head IJH is scanned, as shown in FIG. 20, a heat pulse is applied to the heater of nozzle 1 once for nine reference clocks, a heat pulse is applied to the heater of nozzle 2 once for 12 reference clocks, and a heat pulse is applied to nozzle 3 once for 10 reference clocks. With this operation, the number of ink droplets discharged in the scanning direction is changed for each nozzle, and the ink densities in the pixels of the color filter can be made constant in the scanning direction, as shown in FIG. 20. This makes it possible to prevent density unevenness of each pixel. Correcting ink discharge density in the scanning direction in this manner will be referred to as shading correction.
As methods of reducing density unevenness, the above two methods are known. For example, in a conventional color filter colored in the respective colors in a stripe pattern like the one disclosed in Japanese Patent Laid-Open No. 8-179110, the shading method, which is the latter of the above two methods, is used to adjust the discharge pitch on a pixel array basis so as to adjust the discharge amount for one pixel array. In this striped color filter, a color mixing prevention wall is provided between color pixel arrays to prevent ink of a predetermined color discharged to one pixel array from flowing into an adjacent pixel array of a different color.
In a color filter in which no color mixing prevention wall is provided between color pixel arrays and only a BM (black matrix) is provided as a partition between pixels, unlike a color filter as described above which is colored in a stripe pattern with a color mixing prevention wall being provided between color pixel arrays, when ink is discharged in the form of a line on a pixel array basis, the ink discharged onto the water-repellent BM flows into an adjacent pixel area, resulting in difficulty in managing the amount of ink discharged into each pixel.
That is, it is difficult to control the amount of ink applied into a pixel to a predetermined amount by using a method of adjusting discharge intervals as in the above shading correction.
With an increase in the resolution of color filter pixels, the pixel area tends to decrease. This makes it more difficult to control the amount of ink filled in each pixel.
For this reason, it is important to take new measures to improve the quality of a color filter in association with density unevenness by using the method (bit correction) of making discharge amounts uniform, which is the former method of the above two density unevenness reducing methods.
For example, a technique is proposed in Japanese Patent Laid-Open No. 2000-89019, in which in order to manufacture a color filter without any color unevenness, only nozzles used to print the color filter are caused to discharge ink, the amounts of ink discharged from the nozzles are measured, and the ink discharge amounts of the nozzles are corrected. This is an effective means to eliminate unevenness between pixels by making the ink discharge amounts for printing uniform.
FIG. 22 shows an example of a discharge amount control circuit serving as a discharge amount individual control device for making the discharge amounts of the respective nozzles uniform. In this discharge amount individual control device, a head nozzle driving circuit 304 is provided for each nozzle to adjust the amount of ink discharged from each nozzle. However, in the form in which the head nozzle driving circuits 304 are provided in number equal to the number of nozzles, as the number of nozzles increases, the number of head nozzle driving circuits 304 increases, resulting in increases in circuit size and cost. In the case of industrial printing apparatuses for color filters, which are required to perform mass production, a considerably large number of nozzles are required as compared with home printers, and hence a large number head nozzle driving circuits 304 must be provided. This leads to increases in circuit size, cost, and control load.
As shown in FIG. 22, an electric wire (cable) is used to connect the head nozzle driving circuit 304 to a head 303. If this cable is shorter than an allowable length, noise is superimposed on the cable, or the driving voltage is attenuated. In order to prevent the generation of noise or the attenuation of driving power, the head nozzle driving circuit 304 must be located at a position where the cable connecting the head nozzle driving circuit to the head 303 falls within the allowable length.
This in turn poses a problem in terms of apparatus design, that is, a head nozzle control circuit is too large to be mounted.
In addition, if all nozzles are designed to individually control their discharge amounts, the circuit size of a print control unit 311 also increases.
Furthermore, an increase in overall apparatus size poses problems in terms of difficulty in handling, an increase in consumption power, and an increase in the cost of the apparatus.
In the above description, a color filter has been exemplified as an object to be manufactured. However, the above problems arise not only in the manufacture of color filters but also in a case wherein the amount of liquid applied to a predetermined area (pixel) on a substrate must be controlled to a predetermined amount. For example, such problems arise in a case wherein a predetermined amount of EL (electroluminescence) material liquid is applied from a liquid discharge head (ink-jet head) to a predetermined area on a substrate to manufacture an EL display device. In addition, similar problems arise in a case wherein a predetermined amount of conductive thin film material liquid (liquid containing a metal element) is applied to a predetermined area on a substrate to manufacture an electron-emitting device obtained by forming a conductive thin film on a substrate or a display panel including a plurality of such devices.